Polymeric impurity removal from an aqueous quaternary ammonium salt solution



Unite States Patent O 3,335,162 POLYMERIC IMPURITY REMOVAL FROM ANAQUEOUS QUATERNARY AMMONIUM SALT SOLUTION Charles R. Campbell, Marion J.Mathews Ill, and Roland R. Spiegelhalter, Pensacola, Fla, assignors toMonsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing.Filed May 10, 1965, Ser. No. 454,740 8 Claims. (Cl. 260-459) Thisinvention relates to removing certain organic impurities from an aqueoussolution of a hydrotropic quaternary ammonium salt.

Adiponitrile can be produced by electrohydrodimerizing acrylonitrile.The electrohydrodimerization is carried out in an electrolytic cellhaving an anode chamber and a cathode chamber separated by an ionexchange resin. Acrylonitrile is dissolved in an aqueous quaternaryammonium salt solution. This solution, i.e., catholyte, is thencontinuously circulated through the cathode compartment while a diluteacid solution, i.e., anolyte, is circulated through the anodecompartment. An electrical potential is established between the anodeand cathode sufficient to produce a unidirectional current flow. Thusacrylonitrile is reductively dimerized to adiponitrile at the cathode.

The catholyte is then extracted with acrylonitrile to removeadiponitrile plus a small quantity of certain byproducts, e.g.,propionitn'le, bis-cyanoethylether, hydroxypropionitrile, etc. Theacrylonitrile plus other products is in turn water-Washed to remove anydissolved quaternary ammonium salt. The aqueous quaternary ammonium saltraflinate from the acrylonitrile extraction plus any aqueous quaternaryammonium salt solution obtained from water-washing the acrylonitrileextract are evaporated to the proper quaternary ammonium saltconcentration before the quaternary ammonium salt solution is returnedto the catholyte circulating system.

The salt employed in the electrohydrodimerization of acrylonitrile toadiponitrile must serve two functions: one, the salt must conductelectrical current and, two, it must act to dissolve acrylonitrile inthe aqueous solution of salt, i.e., the salt must be hydrotropic.Quaternary ammonium salts have been found to be admirably suited forsuch purposes. Included among the suitable quaternary ammonium saltsare, generally, tetraalkylammonium sulfates, tetralkylammonium arylsulfonates, and tetraalkylammonium alkyl sulfates. Two specificallyuseful salts are tetramethylammonium toluene sulfonate andtetraethylammonium ethyl sulfate.

Under the described evaporation and recycle procedure a given batch ofquaternary ammonium salt is repeatedly reused. For efiicient celloperation the quaternary ammonium salt solution must be keptsubstantially free of impurities. One impurity formed in theelectrohydrodi- Inerization process is polyacrylonitrile. Most of thismaterial is removable by ordinary filtration or like processes forremoving solids from liquids. Nevertheless, small quantities ofpolyacrylonitrile partially hydrolyze to produce a polymer havingsufiicient carboxylate groups to make it readily soluble in quaternaryammonium salt solution. In one sample of aqueous quaternary ammoniumsalt which had been recycled 25 times partially hydrolyzedpolyacryonitrile was found to be present at a level of about 0.1 percentbased on the solution weight. Even this extremely small quantitydeposits on the cell cathode thereby fouling same and substantiallyreducing the electrolydrodimerization yield. On analysis it wasdetermined that the major portion of the material fouling the cathodeconsisted of deposited polyacrylontrile plus small quantities ofquaternary ammonium salt.

It is a general object of this invention to eliminate or substantiallyreduce cathode fouling in a cell used in the electrohydrodimerization ofacrylonitrile to adiponitrile.

A further object is to substantially remove organic impurities,especially polyacrylonitrile, from an aqueous solution of quaternaryammonium salt.

A still further object is to improve, by reducing all cathode fouling,the product yield in the electrohydrodimerization of acrylonitrile toadiponitrile.

Other objects will become apparent upon reading the followingdescriptive material.

In general, the above objects are achieved by providing a processwherein the above noted partially hydrolyzed polyacrylonitrile isremoved from an aqueous solution of a hydrotropic quaternary ammoniumsalt by means of a procedure including precipitating thepolyacrylonitrile by adjusting the salt concentration of the solution tonot more than about 55 weight percent and adjusting the pH of theaqueous solution to not more than about 7. This precipitated material isthen separated from the aqueous salt solution.

To achieve proper polyacrylonitrile precipitation requires that thetreated quaternary ammonium salt solution have a concentration of notmore than about 55 weight percent and a pH of notlmore than about 7,i.e., be neutral or acidic. There are various ways to achieve theseconditions, depending upon the initial condition of the solution, ofcourse. When the solution is already acidic but the concentration ismore than 55 weight percent then it is only necessary to decrease thesalt concentration to achieve the proper precipitating conditions. Whenboth concentration and pH are above the maximum limits then it isnecessary to lower the salt concentration, i.e., dilute the solution,and to put suflicient acid in the solution to make it acidic.

The order in which the concentration adjustment and pH adjustment takesplace may vary. The solution can be made acidic then have theconcentration adjusted or the concentration can be adjusted, then thesolution can be made acidic. When the salt concentration is greater thanabout 55 weight percent and the pH is greater than 7, i.e., alkaline,the solution can be simultaneously diluted and acidified with a properquantity of an aqueous acid solution.

The most effective precipitation occurs when the pH is not more thanabout 7 and the salt concentration is not more than about 40 weightpercent.

The preferred acids used to acidify the salt solution, because it isdesirable not to contaminate the electrolyte with extraneous acids, arethose whose anion corresponds to the anion of the quaternary ammoniumsalt being treated, for instance, when tetraethylammonium ethyl sulfateis the salt the acid should be ethyl sulfuric acid.

The procedure can be performed in batch form or it can be performed upona continuous stream. The methods for separating the precipitatedmaterial from the salt I solution include filtering, centrifuging, andother like processes well known to persons skilled in the art..

The examples below are set out to more fully illustrate the invention.It is not intended that the examples limit the invention in any mannerwhatsoever.

Example I A quantity of tetramethylammonium toluene sulfonate that hadbeen recycled 25 times through the electrolytic cell employed toelectrohydrodimerize acrylonitrile to adiponitrile was so concentratedby a series of concentration and crystallization procedures until thelevel of organic impurities present in the aqueous salt wasapproximately equivalent to that quantity of organic impurities thatwould have been present if the salt had been recycled about times. Thissalt, having a large quantity of organic impurities therein, was thenemployed as the supporting electrolyte in the catholyte used inelectrohydrodimerizing acrylonitrile to adiponitrile. The catholyteincluded 25 weight percent acrylonitrile (45.9 weight percenttetramethylammonium toluene sulfonate, and 29.1 weight percent water.The pH was 9.0.

Upon conclusion of this test, which lasted for six hours, using highlycontaminated electrolyte the catholyte was extracted with acrylonitrileto recover the adiponitrile and other like organic products. Theremaining tetramethylammonium toluene sulfonate solution was adjusted toa salt concentration of 12 weight percent and the pH was adjusted toabout 1 with p-toluene sulfonic acid. Immediately a precipitateappeared. This precipitate was then filtered from the salt solution andwas determined to be about 0.8 weight percent of the salt solution basedon the dry salt.

The diluted salt solution from which the partially hydrolyzedpolyacrylonitrile had been removed was reconcentrated and had the pHreadjusted, and had enough acrylonitrile added thereto to produce acatholyte substantially equivalent to that described above, i.e., the pHwas adjusted to about 9.0 with tetramethylammonium hydroxide,acrylonitrile concentration to about 25 weight percent,tetramethylammonium toluene sulfonate concentration to about 45.9 weightpercent and about 29.1 weight percent of thecatholyte was water. Thiscatholyte was used in the same cell as was the contaminated electrolyteand was run for the same period of time, six hours. Upon analysis of theacrylonitrile extracted prodnot of electrohydrodimerization the resultsshown in Table l were obtained. All amounts are given as weight percentbased on the total product weight.

TAB LE 1 Products Obtained Contrmlitnated Purified Salt Adiponitrile 72.33 83. 05 Propionitrile r 9. 74 9. 46 BiS-cyanoethylethen. 13. 60 2. 62Hydroxypropionitrile 0. 97 0. 43 Other high boiling impurities 3. 36 4.44

1 Principally Z-cyanoethyladiponitrile.

From the results in Table 1, it is quite apparent that the production ofthe desired product, adiponitrile, is

substantially increased by removal of the partially hydrolyzedpolyacrylonitrile produced in the electrohydrodimerization process.

Example 11 TABLE 2. COMPARATIVE POLYMER REMOVAL RESULTS Salt concentra-Amount of polymer tion, weight pH removed, weight perpercent cent basedon diy salt Thus, it is clear that once the pH of the diluted saltbecomes greater than 7.0 polymer removal becomes negligible. However, ascan be seen from Example 1, best results are obtained when the saltconcentration and pH 5 are lower. than set out in this example.

It can be readily seen that the primary advantage of this invention isthe significant improvement in adiponitrile yield. A second advantage isthe increased useful life span of a particular cathode, i.e., thecathode becomes fouled at a much slower rate when the electrolyte issubstantially free of organic impurities-such as polyacrylonitrile, thusincreasing the periods between cathode cleanmg.

Although the invention has been described by reference to particularembodiments and examples, it is evident that many other embodiments andvariations would be apparentto those skilled in the art to which theinvention pertains. Therefore, the invention should be broadly construedand should only be limitedto the reasonable scope of the appendedclaims.

What is claimed is:

1.A process for. removing partially hydrolyzed polyacrylonitrile from anaqueous solution of a hydrotropic quaternary ammonium salt comprising,in combination, the steps of:

(a) precipitating said polyacrylonitrile by adjusting the saltconcentration of said solution to not more than about 55 weight percentand adjusting the pH of said solution to not more than about 7; and

(b) separating said polyacrylonitrile from said solution.

2. The process of claim 1 wherein the hydrotropic quaternary ammoniumsalt is a tetraalkylammonium aryl sulfonate.

3. The process of claim 1 wherein the hydrotropic sulfate.

4. A process for removing partially hydrolyzed polyacrylonitrile from anaqueous salt solution of ,a hydrotropic quaternary. ammonium saltcomprising, in combination, the steps of:

(a) precipitating said polyacrylonitrile by sequentially,

(1) adding sufiicient water to said aqueous solution to adjust the saltconcentration thereof to not more than about 40 weight percent, (2)acidifying said aqueous solution to adjust the tropic quaternaryammonium salt comprising, in combination, the steps of:

(a) precipitating said polyacrylonitrile by sequentially, (1) acidifyingsaid aqueous salt solution to adjust the pH thereofto notmore than about7;

(2) adding sufiicient water tosaid aqueous salt solution having a pH ofnot more than about 7 to adjust the salt concentration thereof to notmore than about 40 weight percent; and

(b) separating said polyacrylonitrile from said aqueous solution.

6.-A process for removing partially hydrolyzed polyacrylonitrile from anaqueous solution of a hydrotropic quaternary ammonium salt-selected fromthe group consisting of tetraalkylammonium alkyl sulfates andtetraalkylammonium aryl sulfonates used as a supporting electrolyte in aprocess for electrohydrodimerizing acrylonitrile to adiponitrilecomprising, in combination, the 7 steps of:

(a) precipitating said partially hydrolyzed polyacrylonitrile byadjusting the salt concentration of said solution to not more than about40 weight percent and adjusting the pH of said solution to not more thanabout 7; and

quaternary ammonium salt is a tetraalkylammonium alkyl 5 6 (b)separating said polyacrylonitrile from said aqueous References CitedSolution UNITED STATE PAT N s 7. The process of claim 6 wherein thehydrotropic qua- S E T ternary ammonium salt is tetraethylammonium ethyl3,193,481 7/1965 Bauer 204 '73 Sulfate 5 CHARLES B. PARKER, PrimaryExaminer.

8. The process of claim 6 wherein the hydrotropic quaternary ammoniumsalt is tetramethylammonium toluene BERNARD BILLIAN, Assistant Examiner.

sulfonate.

1. A PROCESS FOR REMOVING PARTIALLY HYDROLYZED POLYACRYLONITRILE FROM ANAQUEOUS SOLUTION OF A HYDROTROPIC QUATERNARY AMMONIUM SALT COMPRISING,IN COMBINATION, THE STEPS OF: (A)PRECIPITATING SAID POLYACRYLONITRILE BYADJUSTING THE SALT CONCENTRATION OF SAID SOLUTION TO NOT MORE THAN ABOUT55 WEIGHT PERCENT AND ADJUSTING THE PH OF SAID SOLUTION TO NOT MORE THANABOUT 7; AND (B) SEPARATING SAID POLYACRYLONITRILE FROM SAID SOLUTION.